Radiation therapy is a localised treatment designed to treat an identified tissue target (such as a cancerous tumour) and spare the surrounding normal tissue from receiving doses above specified tolerances thereby minimising risk of damage to healthy tissue. Prior to delivery of radiation therapy, an imaging system can be used to provide a three dimensional image of the target from which the target's size and mass can be estimated and an appropriate treatment plan determined.
Many factors may contribute to differences between the dose distribution determined in the treatment plan and the delivered dose distribution. One such factor is an inconsistency between the patient position at the imaging stage and the patient position in the radiation treatment unit.
Image guided radiation therapy (IGRT) is known. The method involves the use of an imaging system to view target tissues whilst radiation treatment is being delivered to the target tissue. IGRT incorporates imaging coordinates from the treatment plan to ensure the patient is properly aligned for treatment in the radiation therapy device.
Various medical imaging technologies are used to identify target tissues in radiation therapy planning and IGRT. These include (without limitation); Computed Tomography (CT), Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI). MRI is ideal for on-line position verification during radiotherapy, it is able to make fast 2D images of soft tissues with orientation along and perpendicular to the field axis, allowing imaging at critical locations which are defined during the treatment planning procedure. MRI also provides excellent contrast between tissue types giving a sharp image of the target.
The Applicant's prior published international patent application no. WO03/008986 describes a device for use in IGRT which includes the functions of an MRI device in a radiation therapy treatment apparatus and proposes technology for overcoming the problems in doing so.
The large scale of these combined devices will be appreciated. Such devices are typically of the order of 2-3 meters in diameter and they weigh several tons. It will be appreciated they cannot be easily transported or manoeuvred for maintenance and repair.